1. Field of the Invention
This invention relates to the field of semiconductor manufacturing and more specifically to the field of chemical-mechanical polishing (CMP) and to an apparatus for conditioning a polishing pad used in semiconductor manufacturing to polish or planarize a silicon wafer or similar work piece.
2. Description of Prior Art
CMP is used primarily for polishing or "planarizing" the front face of a semiconductor wafer. A silicon wafer typically is fabricated as a disk, and the wafer thereafter is subjected to a masking process in preparation for using it, for example, in a production of integrated circuits.
The masking process causes numerous undesired irregularities on a device surface of the wafer. To remove rough spots and irregularities from the wafer and to produce a planar surface of substantially uniform thickness on the wafer, a CMP process may be used.
FIG. 1 illustrates a cross-sectional view of a typical orbital CMP polisher. During CMP with an orbital polisher 1, a semiconductor wafer ("wafer") 10 is placed onto a polishing pad 12 that has been coated with an abrasive and chemically reactive solution, slurry, which typically is comprised of a colloidal silica. Wafer 10 is held in place and made to bear against a polishing pad surface by a carrier 14. Carrier 14 may be a rigid table or a flexible table comprised of a pliable material. Polishing pad is attached to the top of a flexible or rigid table or platen 16.
Polishing pad 12 typically is constructed in two layers overlying a platen with the less resilient layer on the outer layer of the polishing pad. The hardness and density of the polishing pad 12 depends on the type of material that is to be polished. The parameters of polishing, such as the pressure on the wafer, the rotational speed of the carrier, the speed of the polishing pad, the flow rate of the slurry, and the pH of the slurry are carefully controlled to provide a uniform removal rate, a uniform polish across the surface of the wafer, and consistency from wafer to wafer.
Polishing pad 12 typically is larger than the diameter of the wafer 10 being polished, and wafer 10 generally is kept off-center of the polishing pad to prevent grinding of a non-planar surface into the wafer. Wafer 10 and polishing pad 12 may both axially rotate, or polishing pad 12 may be rotated about a vertical axis while wafer 10 is placed in a confined position. Under either system, slurry may be distributed to the wafer/polishing pad interface through a plurality of holes 18 formed throughout the polishing pad 12. The rotation of polishing pad 12 about the surface of wafer 10 causes the polishing pad to rub against the device surface thereby bringing about abrasive wear of the surface in engagement with the polishing surface.
As a wafer is polished, the slurry and abraded materials tend to glaze the surface of the polishing pad, making the polishing pad slick and reducing the polishing rate and efficiency. Polishing can produce stray particles from the polishing pad material, the wafer itself, or elsewhere. It is important that the polishing pad surface be maintained in planar condition and substantially free of surface irregularities.
One method of countering the glazing or smoothing of the polishing pad surface and achieving and maintaining high and stable polishing rates is to "condition" the polishing pad by removing old slurry particles and abraded particles which develops on the surface. Scraping the polishing pad with a sharp object or roughening the polishing pad with an abrasive material restores the polishing pad's surface, thus increasing the ability of the polishing pad to absorb slurry and increasing the polishing rate and efficiency of the polishing system. During or after conditioning, the polishing pad may be rinsed with water to remove the particles and irregularities loosened during the conditioning process.
Most known polishing pad conditioning systems use an abrasive disk to increase the roughness of the polishing pad and counter the glazing process. In one known conditioning method, as described in U.S. Pat. No. 5,216,843 (Breivogel et al.) and as shown in FIG. 2, a multitude of fine microgrooves 20 are formed in the surface of a polishing pad 12 by pivoting a diamond pointed conditioning block 22 back and forth across an annular area 26 of the polishing pad 12 which contacts the wafer 10. This technique tends to produce nonuniform conditioning, and the effectiveness of the conditioning is limited.
Moreover, because the conditioning block 22 is rigidly connected to conditioning arm 24, operating efficiency is dependant upon the relative motion of the polishing pad 12 and the conditioning block 22, and effective conditioning cannot be achieved without decreasing the polishing rate, and thereby decreasing wafer throughput and increasing fabrication costs. The rigid conditioning assembly cannot achieve maximum uniform conditioning because the conditioning assembly is unable to fully conform to irregularities and unevenness generally present on the surface of the polishing pad at the time of conditioning.
U.S. Pat. No. 5,547,417 (Breivogel et al.) describes a "ball and socket" joint to attempt to achieve uniform contact with polishing pad 18 when irregularities are present on the polishing pad 18. However, this "ball and socket" joint device still provides for a rigid conditioning block with mobility only in the vertical plane. It does not allow for a compliancy in the conditioning block 22 to achieve maximum uniform conformity and contact between the conditioning block 22 and the polishing pad 18 and does not allow the conditioning assembly to conform to minor irregularities in the polishing pad.
Another known method for conditioning a polishing pad uses a large diameter diamond particle covered disk, as described in U.S. Pat. No. 5,456,627 (Jackson et al.). In this method, the large disk is pressed against the polishing pad and axially rotated while the polishing pad rotates. This conditioning technique requires a large diameter disk and has been found less than optimal due to a combination of insufficient surface flatness and inability to track surface variations across the polishing track left in the polishing pad. Moreover, this conditioning device tends to gouge portions of the polishing pad while insufficiently conditioning other portions. The rigidity of the structure of this conditioning device does not allow for uniform conditioning because the disk does not have the flexibility to remain in uniform contact with the polishing pad when irregularities and unevenness are present in the polishing pad.